MEMS resonators allow the integration of accurate timing and frequency reference devices on a silicon chip. This allows major cost and size reductions compared, for example, to conventional quartz oscillator circuits.
MEMS resonators can use extensional (longitudinal) modes, or torsional (bending) modes. Extensional modes are favourable in some applications because they can store more energy than most flexural modes, and are therefore less susceptible to non-linearity.
Extensional-mode—also known as planar-mode—vibration consists of a compression wave in the plane of the material. That is, the physical displacement of the material takes place in directions lying in the same plane as the resonator, alternately stretching and compressing the material. There is little or no displacement (that is, bending or flexion) in the direction transverse to the plane. Torsional mode vibration involves the bending of a structure out of its plane.
In a typical MEMS resonator, the mechanical vibration is excited by electrostatic forces resulting from an electrical signal applied via one or more electrodes spaced a small distance apart from the resonator. The vibration gives rise to a changing capacitance, which can be detected either via a separate output electrode (or electrodes) or via the body of the resonator itself.
In a capacitive read-out arrangement, in order to have a measurable signal, the change in capacitance during movement of the MEMS resonator needs to be large enough. This is usually achieved by making the MEMS structures high (using ˜10 μm Silicon-on-Insulator (SOI) layers) and wide. It is therefore very hard to scale these devices down to smaller sizes (and higher frequencies) without decreasing the signal (the admittance) too much.
One method to overcome this disadvantage is to make use of a piezo-resistive read-out arrangement, as disclosed by J. T. M. van Beek et al, “Scalable 1.1 GHz fundamental mode piezo-resistive silicon MEMS resonator”, IEDM 2007 pages 411-414. The read out method involves sending a small current through the resonating devices. The mechanical movement of the device will induce mechanical stretching which in turn will change the resistance of the current path (by the piezo-resistive effect) and register as an electrical signal.
When the input signal is at or near the resonant frequency of the device, resonance occurs, effectively amplifying the output signal at this frequency by the gain factor Q of the device.
The size of a MEMS resonator is inversely related to the resonant frequency. Current sizes are sufficiently small to make Radio-Frequency (RF) resonators realizable using this technology. This makes MEMS technology an exciting prospect for next generation wireless communication devices. However, to prove a viable alternative to conventional resonators, MEMS devices must match or exceed their performance. The devices must resonate in predictable and stable modes and be highly efficient transducers of electrical/mechanical energy.
One of the disadvantages of the piezo-resistive read-out method is that a small current is needed through the resistor in order to measure the change in resistance. The read out method therefore consumes electrical power.